FIG. 1 illustrates a conventional optical shunt device for an optical communication system disclosed in Japanese Patent Laid-Open No. 62-73225 light, in which FIG. 1a shows a state in which a prism is inserted into a light path and FIG. 1b shows a state in which the prism is moved out of the light path. FIG. 2 is a schematic diagram illustrated a conventional light transmitting system.
Hereinafter, "communication node" means a shunt device or a shunt station and may be simply referred to as "station".
In FIG. 1, reference numeral (1) designates an optical switch for changing a light path through which an optical signal is supplied, (2) is a highly transparent optical prism movably mounted in the optical switch (1). The transparent prism (2) is inserted into the light path so that, when any of the stations of the system fails, the optical signal is allowed to pass through that particular station. (3) and (4) are up light-input end and down light-input end of the optical switch (1), respectively, (5) and (6) are down light-output end and up light-output end of the optical switch (1), respectively.
In FIG. 2, (7) to (10) are communication systems (stations), wherein (7) is B-station, (8) is C-station, (9) is D-station and (10) is A-station. (11) is an optical fiber for transmitting a light signal.
The conventional optical communication system is constructed as above described, and the optical switch (1) employing the transparent prism (2) as a dividing communication unit of the optical communication system. The operation of this will be described below.
The light incoming into the up light-input end (3) of the optical switch (1) passes through the transparent prism (2) and exits from the down light-output end (5) (See FIG. 1(a)). This light passes through the B-station (7) of FIG. 2, for example, enter into the down light-input end (4) again and, after passing through the transparent prism (2), transmitted from the up light-output end (6) to the optical switch (1) of the C-station (8) which is the next station. In this manner the light signal from the A-station (10) is transmitted to the D-station (9) through the optical fiber (11) and successively through the respective stations. In normal operation, the B-station (7), the C-station (8) and the D-station (9) each amplifies the light signal to compensate for the attenuation of the light signal due to the absorption and the scattering of light within the light path.
If a failure occurs in either of the above stations, the transparent prism (2) in the optical switch (1) of that particular station is moved by a drive unit (not shown) to allow the light entered into the up light-input end (3) of the optical switch (1) to directly emit from the up light-output end (6) (see FIG. 1(b)). Thus, the light is transmitted to the next normal station without being shunted or amplified.
Thus, in the conventional system, a loop comprising the A-station (10).fwdarw.the B-station (7).fwdarw.the C-station (8).fwdarw.the D-station (9).fwdarw.the A-station (10) in which the light signal is transmitted in one way is formed.
In the conventional optical communication system as above described, when a fault occurs in any of the stations connected to construct an optical transmission system, the light signal is transmitted to the next station by allowing it to pass through that particular station by the optical switch (1).
However, in the conventional system of this kind, an optical transparent prism (2) which is expensive and difficult to mass-produce is used in the optical switch (1) which is a dividing communication element of the optical communication system.
Also, it is necessary to connect each station to form a one-way loop in order to construct the optical transmission system, so that a directionality is required in the light signal transmission direction.
Further, when a fault occurs in any of the stations, a drive mechanism for moving the transparent prism (2) is neccessary, making the system itself expensive and large-sized.
Therefore, in the optical communication system of this kind, an optical shunt communication system of a simple structure which does not use expensive optical transparent prisms and yet capable of transmitting and receiving the light signals in either directions is desired to be developed.